The present invention relates to a swept frequency signal generator which is employed as a swept frequency local oscillator, for instance, in a spectrum analyzer and the oscillation frequency of which is swept by a sweep voltage.
For example, in a spectrum analyzer, a sweep voltage is generated and provided to a voltage controlled oscillator (hereinafter referred to as the VCO) to sweep its oscillation frequency and the swept frequency signal is used as a local signal for receiving an input signal for displaying on the screen of a cathode ray tube its frequency component or the frequency characteristic or the like of a circuit to be measured.
It is required that the oscillation frequency of the VCO accurately sweep a preset frequency range. To meet this requirement, it has been proposed to compare sweep start and stop voltages of the sweep voltage from a sweep voltage generator with set values and to control the sweep start and stop voltages to agree with the set values.
Even if the sweep voltage is accurately defined, however, it is unwarranted to expect that the oscillation frequency of the VCO which is controlled by the sweep voltage sweeps a predetermined frequency range. That is to say, in the VCO using a YIG, since a magnetic circuit is controlled, the frequency sweep characteristic has a hysteresis and nonlinearlity owing to the magnetic circuit, resulting in the defect of low setting accuracy of the sweep start and stop frequencies.
A solution to the abovesaid defect has been proposed in U.S. Pat. No. 4,130,808 issued Dec. 19, 1978 and entitled "Phase Lock Stabilized Swept Frequency Signal Source". According to this U.S. patent, prior to the start of each sweep, the VCO output and the output of a high stability reference oscillator are phase compared by a phase comparator and the phase compared output is held in a sample hold circuit and, at the same time, the held output is supplied as a control signal to the VCO via a summing circuit to establish a phase locked loop, causing the oscillation frequency of the VCO to agree with the oscillation frequency of the reference oscillator. Then the output of the phase comparator is cut off and the outputs of the sample hold circuit and the sweep voltage generator are summed by the summing circuit, the output of which is provided to the VCO to control it. In this way, the sweep start frequency is made to accurately coincide with the oscillation frequency of the reference oscillator. With this conventional technique, however, it is unwarranted to expect that the sweep stop frequency accurately agrees with a desired frequency and it is likely to deviate therefrom.